The present invention relates to an optical transmission system, and a transmitter and/or receiver used in optical communication, in particular, relates to such a system which reduces optical signal bandwidth with return-to-zero (RZ) signal, and base-band electrical signal bandwidth in a transmitter.
Recently, an optical amplifier having high output power and wideband characteristics has been used in an optical transmission system, and input fiber launched power in an optical transmission line exceeds 10 dBm. As a result, the Kerr effect in which refractive index in an optical fiber is modulated by an input optical signal itself occurs, and therefore, an optical signal is phase modulated, the optical modulation spectrum is spread, and the waveform is subject to be distorted due to chromatic dispersion in an optical fiber. Further, in a wavelength division multiplex system, waveform and S/N ratio are degraded because of non-linear cross talk between channels.
It is recognized that above problems depend upon format of signals, and RZ (return-to-zero) signal in which each bit has equal pulse width with each other is preferable to NRZ (non-return-to-zero) signal since equalization of waveform distortion due""to non-linear effect after fiber transmission is easy in RZ signal.
For instance, in an inline-repeatered system in which the dispersion of 1.3 xcexcm zero dispersion optical fiber line is compensated for each repeater section, it is estimated in a simulation that the regenerative repeater section for RZ signal may be three times as long as that for NRZ signal (reference; D. Breuer et al, xe2x80x9cComparison of NRZ and RZ-Modulation format for 40 Gbit/s TDM Standard-Fiber Systemxe2x80x9d, IEEE Photon. Technol. Lett. vol 9, No. 3, pp. 398-400, 1997). Further, an experimental report (R. M. Jopson et al, xe2x80x9cEvaluation of return-to-zero modulation for wavelength division multiplexed transmission over conventional single-mode-fiberxe2x80x9d, Tech. Digest of Optical Fiber Comm. Conf, ""98 FEI, pages 406-407, 1998) shows that RZ signal may have higher power for each channel than NRZ signal in 10 Gbit/s 8-waves WDM transmission system. Further, another experimental report (A.Sano et al, IEE Electronics Letters vol 30, pages 1694-1695, 1994) shows that phase modulation synchronized with transmission data effectively suppresses SBS (Stimulated Brillouin Scattering) higher fiber launched power.
Therefore, it is preferable to use RZ signal format in long distance optical transmission system.
A prior optical transmission system with RZ (return-to-zero) signal format is shown in FIGS. 28 and 29.
In FIG. 28, an input NRZ signal (non-return-to-zero) is applied to a NRZ/RZ converter 51 which converts NRZ electrical signal format to RZ electrical signal format. An output signal of the converter in RZ signal format is applied to an RZ optical modulator 50 through an amplifier 52 which amplifies a RZ electrical signal. The optical modulator 50 modulates CW (continuous wave) optical signal from an optical source 5 with a RZ electrical signal from the amplifier 52, and provides modulated optical signal.
In FIG. 29, an input electrical NRZ signal is applied to a modulator driver 62 which amplifies the electrical signal. The output of the modulator driver 62 is applied to a first NRZ optical intensity modulator 60 which modulates continuous wave (CW) from an optical source 5 with the NRZ signal from the amplifier 62. An output of the first modulator 60 is applied to a second optical intensity modulator 61 which modulates the input NRZ signal of the same with electrical sinusoidal wave of an output signal of a clock modulator driver 63. The clock modulator driver 63 provides a clock signal with frequency B (Hz) (B; transmission symbol rate) which is synchronized with an input NRZ electrical signal. Thus, a final RZ optical signal is obtained at the output of the second modulator 61. This prior art is shown in, for instance, A. Sano et al. IEEE electronics Letters vol. 30, pages 1694-1695, 1994.
Another prior art is shown in JP patent laid open 254673/1996 (which corresponds to U.S. Pat. No. 5,625,722; xe2x80x9cMethod and Apparatus for Generating data encoded Pulses in Return-to-zero Formatxe2x80x9d), in which periodical transmittance of a Mach-Zehnder type optical intensity modulator is used in full-wave rectifying characteristics using amplitude folding electrical-optical response of the Mach-Zehnder type optical intensity modulator, and binary NRZ electrical signal is converted to RZ optical signal. An input binary NRZ electrical signal is encoded in a pre-code circuit to produce coded NRZ electrical signal, then, two copies of the NRZ signal are produced, and one of the NRZ signal is logically inverted. RZ signal is generated by operating a Mach-Zehnder type optical intensity modulator with these differential coded NRZ electrical signal.
Further, the following three documents show how to produce optical clock pulses from clock electrical signal.
(1) K. Iwatsuki et al. xe2x80x9cGeneration of transform limited gain-switched DFB-LD pulses  less than 6 ps with linear fiber compression and spectral windowxe2x80x9d, Electronics Letters vol. 27, pp 1981-1982, 1991. In this document, a gain switch semiconductor laser is used as a generation element.
(2) M. Suzuki, et al, xe2x80x9cNew application of sinusoidal driven InGaAsP electroabsorption modulator to in-line optical gate with ASE noise reduction effectxe2x80x9d, J. Lightwave Technol. 1992, vol. 10 pp. 1912-1918. This document shows how to modulate CW optical signal generated by a semiconductor laser by using an electro-absorption type external modulator.
(3) K. Sato et al, xe2x80x9cFrequency Range Extension of actively mode-locked lasers integrated with electroabsorption modulators using chirped gratingxe2x80x9d J. of selected topics in quantum electonics vol. 3, No. 2, 1997, pp. 250-255. This document shows an integrated mode-locked semiconductor laser. But, these three papers do not describe modulation means.
However, above prior art have the disadvantage that an output RZ optical signal has optical bandwidth larger than 4B when transmission rate is B (bit/s). That figure is twice as large as the bandwidth of NRZ optical signal. Therefore, an output RZ signal in prior art is subject to waveform distortion because of chromatic dispersion in an optical transmission fiber as compared with a NRZ optical signal.
FIG. 30 shows NRZ optical signal spectrum in the prior art, and FIG. 31 shows RZ optical signal spectrum in the prior art. It should be noted in FIGS. 30 and 31 that RZ optical signal has bandwidth twice as large as that of NRZ optical signal.
Further, in the prior art of FIG. 28, a NRZ/RZ converter 51, an amplifier 52 and an optical modulator 50 must have the operational bandwidth twice (DC through 2B Hz) as large as the bandwidth (B) which is required for NRZ electrical signal. Thus, the higher the transmission rate is, the more difficult the design of a circuit is.
Further, in the prior art of FIG. 29, two optical modulators 60 and 61 are connected in series. Therefore, in order to keep S/N ratio of a resultant RZ signal output to be the same as that of NRZ optical signal, an output of an optical source 5 must be increased by 6-9 dB so that optical loss and modulation loss for one stage of an optical modulator are compensated, and therefore, an optical source must provide high output power. Further, a phase control circuit 64 is essential to adjust the modulation phase between the NRZ optical signal and the synchronization clock signal.
Further, in above prior art, an output RZ modulated optical signal has fine spectrum at fcxc2x1nxB (Hz) (n is an integer), where fc is carrier frequency of continuous wave light. Therefore, when signal power applied to an optical fiber exceeds 7 dBm, the input fiber launched power to a dispersion shifted fiber is limited because of Stimulated Brillouin Scattering (SBS). Therefore, an external linewidth modulation circuit 53 is necessary to enlarge linewidth of optical carrier for SBS suppression and increase allowable input power.
Further, RZ optical signal has optical carrier frequency component (fc) as shown in FIG. 31. Therefore, when RZ optical signal spectrum components are equal or higher than threshold density of Stimulated Brillouin Scattering (SBS), those high density spectrum components are back-scattered by SBS, and waveform is distorted. This is described, for instance, in H. Kawakami et al, xe2x80x9cOvermodulation of intensity modulated signal due to Stimulated Brillouin Scatteringxe2x80x9d, Electron. Lett. vol. 30, No. 18, pp. 1507-1508. When those RZ signals are wavelength division multiplexed (WDM), a portion having high optical spectrum density includes fourwave mixing (FWM), and cross talk due to pump depletion.
Further, in a prior RZ electrical driven amplifier which amplifies electrical signal directly (FIG. 28), when a modulator driver is an AC-coupling type, DC level of drive signal is fluctuated due to mark ratio of modulation signal, thus, output dynamic range of a driver circuit must be twice as large as that with fixed mark ratio. Further, a control circuit is necessary for controlling bias point of an optical intensity modulator according to mark ratio.
As described above, the prior art has the disadvantages that (1) since bandwidth for RZ optical signal is twice as large as that of NRZ optical signal, RZ optical signal is more sensitive to the distortion due to the chromatic dispersion of an optical fiber, (2) bandwidth required for an electrical circuit operating with RZ signals is twice as large as that for NRZ signal, (3) since input power to a fiber is limited due to SBS, an external circuit for enlarging optical carrier frequency linewidth is essential, (4) cross talk is generated due to four wave modulation (FWM), and that (5) DC level of drive signal is fluctuated because of mark ratio change.
An object of the present invention is to overcome the disadvantages and limitations of a prior optical transmission system by providing a new and improved optical transmission system.
It is an object of the present invention to provide an optical transmission system in which optical spectrum bandwidth for RZ optical signal is half as that of the prior art, and optical signal is less degraded due to chromatic dispersion in an optical fiber.
It is also an object of the present invention to provide an optical transmission system in which electrical signal bandwidth required for an electrical circuit and an optical intensity modulator is approximately equal to transmission rate B.
It is also an object of the present invention to provide an optical transmission system in which limitation of optical input fiber launched power due to SBS is essentially reduced.
It is also an object of the present invention to provide an optical transmission system in which output power of an optical source may be reduced.
It is also an object of the present invention to provide an optical transmission system in which cross talk because of four wave mixing (FWM) is eliminated.
It is also an object of the present invention to provide an optical transmission system in which no D.C. level fluctuation due to mark ratio change occurs.
It is also an object of the present invention to provide an optical transmission system in which input-pattern dependent inter-symbol interference is reduced in an optical transmitter/receiver.
The above and other objects are attained by an optical transmission system having an input terminal which receives NRZ (non-return-to-zero) electrical signal, and electrical-optical conversion means which receives said NRZ electrical signal and converts the same to RZ (return-to-zero) optical signal, wherein said electrical-optical conversion means comprises; pre-code means receiving an NRZ electrical signal in complementary form and providing a pre-coded signal which is an exclusive-OR signal of said NRZ electrical signal and delayed signal of said NRZ electrical signal by one bit; differentiation means for providing differentiated ternary level pulses having first level, second level and third level at a rising edge, a duration between a rising edge and a falling edge and a falling edge of said pre-coded signal, respectively, so that polarity of a pulse at said rising edge is opposite to that at said falling edge; and an optical intensity modulation means for providing an optical signal according to said differentiated pulses so that an optical phase corresponding to the first level of the differentiated pulse is opposite to an optical phase corresponding to the third level of the differentiated pulse. The optical output is turned off at the second level of input differentiated pulse.
Preferably, said optical intensity modulator comprises a Mach-Zehnder optical intensity modulator.
When an input NRZ signal is not in complementary form, but single end form, the similar operation is obtained if a pre-code means provides an output signal in complementary form.
As the present invention pre-codes an input NRZ electrical signal, then, it is differentiated, the pre-coded signal is D.C. balanced signal with no D.C. component, and in ternary form. Therefore, an electrical circuit and an optical intensity modulator are not required to operate to amplify and/or modulate in baseband signal component including D.C., and the required operational bandwidth is equal to transmission rate B. Further, as a differentiated signal includes no D.C. component, no D.C. level fluctuation occurs irrespective of mark ratio. Further, as one stage of optical intensity modulator is used, output power requested for CW optical source is decreased, as compared with the scheme in FIG. 29.
Further, as a differentiated signal is used, an optical RZ signal has no optical carrier frequency component irrespective of mark ratio, and RZ optical signal spectrum density is lower than that of the prior art. Therefore, RZ optical signal power in which the maximum spectrum density is equal to threshold value of Stimulated Brillouin Scattering in the present invention is higher than that of the prior art. Further, as the spectrum density is low, when wavelength division multiplexed signal is transmitted close to zero dispersion wavelength region, cross talk due to four wave mixing (FWM) is reduced, whereas said cross talk is serious in a prior RZ and/or a prior NRZ transmission system.
Further, according to the present invention, phase of an optical pulse is inverted pulse by pulse, therefore, when multi-path fading occurs due to polarization mode dispersion in a transmission line, phases of overlapped portion of pulse edges are opposite to each other. Therefore, in intensity modulated signal, intensity at pulse edges is cancelled by interference, and no inter-symbol interference occurs.
According to another embodiment of the present invention, an optical transmission system has an input terminal which receives NRZ (non-return-to-zero) electrical signal, and electrical-optical conversion means which receives said NRZ electrical signal and converts the same to RZ (return-to-zero) optical signal, wherein said electrical-optical conversion means comprises; clock electrical signal generation means for providing clock electrical signal with which said NRZ electrical signal is synchronized; clock pulse optical source receiving said clock electrical signal and providing optical clock pulse synchronized with said clock electrical signal; pre-code means receiving an NRZ electrical signal in signal-end form and providing a pre-coded signal in complementary form which is an exclusive-OR signal of said NRZ electrical signal and delayed signal of said exclusive-OR signal by one bit; a pair of complementary logical product means for providing complementary logical product signals each of which is a logical product of said NRZ electrical signal and one of pre-coded differential NRZ signals; an optical intensity modulation means having two modulation sections electrically isolated with each other and arranged in series along optical path for modulating intensity and phase of said optical clock pulse independently with said complementary logical product signals so that phase of an optical output signal is inverted for each mark code.
According to still another embodiment of the present invention, an optical transmission system has an input terminal which receives NRZ (non-return-to-zero) electrical signal, and electrical-optical conversion means which receives said NRZ electrical signal and converts the same to RZ (return-to-zero) optical signal, wherein said electrical-optical conversion means comprises; clock electrical signal generation means for providing clock electrical signal with which said NRZ electrical signal is synchronized; clock pulse optical source receiving said clock electrical signal and providing optical clock pulse synchronized with said clock electrical signal; pre-code means receiving an NRZ electrical signal in a single-end form and providing a pre-coded signal in complementary form which is an exclusive-OR signal of said NRZ electrical signal and delayed signal of""said exclusive OR signal by one bit; a pair of complementary logical product means for providing logical product signals in complementary form, each of said logical product signals being a logical product of said NRZ electrical signal and one of pre-coded complementary NRZ signals; a pair of power sum means for providing power sum of one output of one of said complementary logical product means and a complementary output of the other complementary logical product means, and an optical intensity modulation means for modulating said optical clock pulse with outputs of said power sum means so that phase of an optical output signal corresponding to first level of said power sum signal is inverted from phase of an optical output signal corresponding to third level of said power sum signal.
According to still another embodiment of the present invention, an optical transmission system has an input terminal which receives NRZ (non-return-to-zero) electrical signal, and electrical-optical conversion means which receives said NRZ electrical signal and converts the same to RZ (return-to-zero) optical signal, wherein said electrical-optical conversion means comprises; a first optical intensity modulation means for modulating continuous optical signal with optical clock signal; a second optical intensity modulation means for modulating an output of said first optical intensity modulation means with said NRZ electrical signal; pre-code means receiving an NRZ electrical signal and providing a pre-coded signal which is an exclusive-OR signal of said NRZ electrical signal and delayed signal of said exclusive-OR signal by one bit, and an optical phase modulation means for modulating phase of an output of said second optical intensity modulation means with an output of said pre-code means so that phase of an optical pulse of said second optical intensity modulation means is inverted alternately pulse by pulse.